Noninteracting control of moisture and fiber content of fibrous sheet during manufacture

ABSTRACT

MOISTURE AND FIBER CONTENT OF A FIBROUS SHEET ARE CONTROLLED DURING MANUFACTURE BY MEASURING THE SHEET MOISTURE DIWNSTREAM OF A DRYER IN THE MANFUACTURING MACHINE, AS WELL AS THE SHEET FIBER CONTENT. THE FIBER CONTENT AND MOISTURE MEASUREMENTS ARE COMBINED TO CONTROL THE DRYER DRYING RATE IN SUCH A MANNER THAT CHANGES IN THE RATE AT WHICH FIBER IS FED TO THE MACHINE ARE COMPENSATED FOR BY CHANGES IN THE DRYER DRYING RATE SO THAT CHANGES IN THE MOISTURE CONTENT OF THE SHEET ARE GREATLY MINIMIZED. THE RATE OF FIBER FLOW IS CONTROLLED IN RESPONSE TO AN ERROR   SIGNAL FOR THE SHEET FIBER CONTENT AND THE RATE AT WHICH FIBER IS FED INTO MACHINE.

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July 11, 1972 J. 5. RICE 3,676,295

NONINTERACTING CONTROL OF MOISTURE AND FIBER CONTENT OF FIBROUS SHEETDURING MANUFACTURE Filed Sept. 12, 1969 2 Sheets-Sheet L COM PUTEQM/L/E/VTOE, Jwis 5. F/(E July 11, 1972 J. 5. RICE 3,676,295

NONINTERACTING CONTROL OF MOISTURE AND FIBER CONTENT OF FIBROUS SHEETDURING MANUFACTURE Filed Sept. 12, 1969 2 Sheets-Sheet 2 B.W. 4 6. 4 QREC-1. Q 57 ABDBW 4% x Ave.

A? STOCK M %g)' T VALVE 5V- SET v f 53 STEAM P STEM VALVE $ET R VALVEPSET H6. 5 TO 33 8 aw. i J X 6 es.

so 90 x "l "l as 89 2 TO M V PAC 64 3e 72 //vv. A/70/e,

JA'MES 5 F/(E JTTOF/ fYS United States Patent US. Cl. 162-198 9 ClaimsABSTRACT OF THE DISCLOSURE Moisture and fiber content of a fibrous sheetare controlled during manufacture by measuring the sheet moisturedownstream of a dryer in the manufacturing machine, as well as the sheetfiber content. The fiber content and moisture measurements are combinedto control the dryer drying rate in such a manner that changes in therate at which fiber is fed to the machine are compensated for by changesin the dryer drying rate so that changes in the moisture content of thesheet are greatly minimized. The rate of fiber flow is controlled inresponse to an error signal for the sheet fiber content and the rate atwhich fiber is fed into the machine.

The present invention relates generally to control systems and methodsfor machines fabricating fibrous sheets and more particularly to acontrol system and method wherein sheet fiber and moisture content arecontrolled in a manner so that moisture variations normally induced inthe sheet due to changes in the rate at which fiber is fed to themachine are compensated by controlling a dryer so that moisture in thesheet remains substantially constant.

Machines for fabricating fibrous sheets, such as paper, generallyinclude a stock valve for controlling the flow of a fiberwater mixtureinto the machine and a dryer for removing water from the sheet after ithas been formed. Concomitant or simultaneous control of the sheetmoisture and fiber content in response to measurements of sheet basisweight, a parameter indicative of total sheet weight per unit area, andmoisture has been proposed. Generally, the approach has been to controla steam pres sure valve for the dryer in response only to a moisturesignal and the stock valve only in response to total basis weight orbone dry basis weight content, a parameter indicative of dry fiberWeight, i.e., total weight minus moisture. It has been found, however,that such concomitant controls do not enable the stock valve and dryerto be controlled on a noninteracting basis. Instead, corrections made tothe sheet fiber Weight affect the sheet moisture properties and a sheethaving the desired moisture is not produced. If perfect noninteractionoccurred, a change in the stock valve would have no efiect on moistureof the sheet, and hence a noninteracting controller for a paper makingmachine is one in which a change in fiber flow rate would not be allowedto have an effect on moisture of the sheet, except for transientphenomena. It has also been found that variations in the sheet totalbasis weight, if corrected only by controlling the stock valve, cause asheet having a moisture content different from a desired or target valueto be produced.

A mathematical analysis of the dry fiber content per unit area or bonedry basis weight, as well as the moisture properties, of a sheetverifies the previously experimentally noted results. In particular, thebone dry basis weight, BDBW, and moisture M, of a paper sheet can berelated to the pressure, P, of steam in the dryer and flow rate, Q, offiber or stock through the stock valve in accordance with the functionalrelationships:

3,676,295 Patented July 11, 1972 'ice In response to incremental changesin the form of corrections to the fiber flow rate (AQ) and steam dryerpressure (AP), changes in bone dry basis weight (ABDBW) and moisture(AM) occur and can be respectively represented as:

BBDBW oBDBW A ABDBW= AQ-i- OP P and 6M 5M AM=XQAQ+6FAP (3) where:

BBDB W/6Q is the rate of change of bone dry basis weight with respect tofiber flow;

BBDBW/EP is the rate of change of bone dry basis Weight with respect tosteam dryer pressure;

aM/aQ is the rate of change of moisture with respect to fiber flow; and

QM/6P is the rate of change of moisture with respect to steam dryerpressure.

For any grade and type of paper being manufactured, the partialsEBDBW/BP, BM/ZBQ and BM/BP can be assumed constant, while the partialaBDBW/aQ can be approximated as BDBW/Q. Since the fiber fed into amachine is controlled exclusively by the stock valve and the dryer hasno effect thereon, the partial The partials EM/BQ and 'dM/aP arecoefiicients which can be experimentally determined for each machine andmerely vary between grades and types of paper for most practicalpurposes and are thereby validly assumed constant for a particular setof machine parameters.

In accordance with the present invention, Equations 2 and 3 are solvedfor the values of AQ and AP to provide values for changes in the fiberflow rate and steam pressure necessary to fabricate a fibrous sheetwherein bone dry basis weight and moisture do not substantially interactwith each other or have a minimal interaction and are reached withoutover or undershoot. The dryer steam pressure and fiber stock flow rateare varied in a coordinated manner so that changes in the fiber flowrate produce very small errors in the moisture content. In other Words,if a sheet being manufactured does not have the correct fiber contentbut does have the desired moisture content, the solution of Equations 2and 3 for AQ and AP gives the amount of correction to fiber flow rateand dryer drying rate to enable the fiber error to be corrected with aminimum change in the desired sheet moisture content. If only the fiberflow rate were corrected in response to a fiber error signal, withoutcontrol of the drying rate, the total sheet weight would be excessivelychanged to produce overshoot of the moisture control.

Solving Equations 2 and 3, with appropriate substitutions, yields:

For a particular steam dryer pressure, P,

W2 eM BBDBW BDBW so that Equation 5 can be rewritten in accordance with:

6M AP -ABDBW+ AM According to a feature of the present invention, thesetting of the stock valve is coordinated with the setting of the dryersteam valve in response to an error in the actual bone dry basis weightrelative to the target or desired value therefor, multiplied by the fiowof fiber through the stock valve divided by the actual bone dry basisweight. The ratio of the actual bone dry basis weight to the actualfiber stock flow is approximately equal to the coefiicient BBDBW/EQ inEquation 2, a variable which may be subject to greater changes than theremaining partial derivative coefiicients of Equations 2 and 3.

I am aware of an article entitled Designing and Tuning DigitalControllers, written by E. B. Dahlin et al.,

in the July 1968, Instruments and Control Systems, pp.

11-15, wherein there is disclosed a system for preventing interactionbetween the fiber and moisture content of a sheet produced by a papermaking machine. The present invention, however, provides resultssuperior to those of systems developed in accordance with the Dahlin etal. article. In the Dahlin et al. article there is disclosed, in verybroad terms, a system for controlling the dryer steam valve, as well asthe stock valve, of a paper making machine in response to signalsindicative of moisture and total basis weight of the formed sheet. Inthe present invention the steam valve is controlled in response to themoisture and bone dry basis weight, i.e., dry fiber content, of thesheet. By controlling the steam valve in response to moisture and bonedry basis weight, rather than total basis weight, the stock valveposition can be controlled only in response to a bone dry basis weighterror signal. In systems relying upon control in response to total basisWeight and moisture, as disclosed by Dahlin et al., the stock valve, aswell as the dryer steam valve, must be controlled in response to bothmoisture and total basis weight signals in addition to many parametersindicative of the machine characteristics. In particular, the stock andsteam valves are both controlled in response to error signals for totalbasis weight and moisture, as well as machine parameters commensuratewith rate of change of moisture with respect to dryer pressure, totalbasis weight with respect to dryer pressure, total basis weight withrespect to stock flow rate, and rate of change of moisture with respectto stock flow. In the system developed by Dahlin et al., it wasapparently felt that each of these machine parameters should bedetermined while the paper machine is in actual operation in response toperturbations actually applied to the machine. At the time suchperturbations are applied to the machine no control is being performedand variations from property target values are intentionally introducedinto the sheet being produced. Thereby, errors in sheet properties canarise due to two causes while the machine parameters are beingdetermined. In the system of the present invention, where control is inresponse to bone dry basis weight and moisture, only three machineparameters are employed. As indicated supra, these parameters are rateof change of moisture with respect to bone dry basis weight, rate ofchange of moisture with respect to dryer pressure and rate of change ofbone dry basis weight with respect to stock flow. For any particulargrade of paper, I have found that the two moisture rates of change canbe determined with sufficient accuracy on an a priori basis to enableaccurate closed loop control to be performed and that EBDBW/BQ can beaccurately approximated as the ratio of actual bone dry basis weight tostock flow. Therefore, a paper machine controlled in accordance with thepresent invention does not require time consuming periodic perturbation,which can cause errors in sheet properties, to determine machinecharacteristics. Further, problems of controller implementation aresubstantially reduced because fewer terms are required and the need toconstantly update all of the machine parameters does not exist.

I have been informed that systems actually built utilizing the totalbasis weight approach described in the Dahlin et al. article are subjectto problems of transients and poor regulation. It is likely that theseproblems arise because the stock valve is controlled in response to twoerror signals, viz: total basis weight and moisture, which may interactto produce overshoot or undershoot of moisture and basis weight. It hasbeen found in a system actually constructed in accordance with thepresent invention that these problems are substantially reduced,apparently because only the bone dry basis weight error signal controlsthe stock valve and is employed in combination with the moisture errorto control the steam valve. Controlling in response to the bone drybasis weight error is significant even though moisture and total basisweight measurements are usually combined to derive a signalindicative-of bone dry basis weight. To the casual observer, it mightappear that the bone dry basis weight error is merely a combination ofthe moisture and total basis weight errors. This is not the case,however, because of the multiplicative relationship between total basisweight and moisture. To derive an accurate indication of bone dry basisweight error, bone dry basis weight should be determined by combiningthe moisture and total basis measurements and subtracting the calculatedbone dry basis weight value from a set point or target value therefor.

In a system of the type disclosed by Dahlin et al., the total basisweight and moisture signals resulting from a scan of the gauge acrossthe sheet width are averaged to derive signals which are compared withtarget values therefor to derive the total basis weight and moistureerrors. If the sheet being scanned has relatively great variations indry fiber weight across its width, these may not be reflected in thecontrol actions derived from the total basis weight and moisture errorsbecause of the multiplicative nature by which total basis weight andmoisture are combined to obtain bone dry basis weight. In accordancewith another aspect of the present invention, total basis weight andmoisture measurements made in each of a plurality of cross-sheet zonesin response to scanning gauges are combined to derive a measure of bonedry basis weight in each zone. The bone dry basis weight indications forthe several cross-sheet zones are averaged to derive an accurateindication of sheet fiber weight across the sheet width scanned toenable relatively error free noninteracting control of the sheetmoisture and fiber content to be attained.

It is accordingly an object of the present invention to provide a newand improved system and method for concomitantly controlling themoisture and fiber content of a fibrous sheet during manufacture.

Another object of the present invention is to provide a new and improvedsystem for and method of concomitantly controlling the fiber andmoisture content of a fibrous sheet in a coordinated manner so thatchanges in the rate at which fiber is fed to the machine do notsubstantially afiect the sheet moisture.

A further object of the present invention is to provide a system for andmethod of manufacturing a fibrous sheet wherein changes in the rate atwhich fiber is fed to the machine do not substantially affect the sheetmoisture content and only a single variable indicative of a sheetparameter is required to control the fiber flow rate.

Still another object of the present invention is to provide a new andimproved system for the method of controlling the fiber and moisturecontent of a fibrous sheet in resopnse to signals derived from scanninggauges by concomitantly controlling a dryer steam valve and fiber stockvalve so that changes in the rate at which fiber is fed to the machinedo not substantially affect the moisture content of the sheet.

Still another object of the invention is to provide a noninteractingsystem for controlling the moisture and fiber content of a fibrous sheetduring formation wherein the stock valve setting is controlled inresponse to measured dry fiber weight and the dryer steam valve iscontrolled in response to measured dry fiber weight and moisture.

Still another object of the present invention is to provide anoninteracting system for controlling the moisture and fiber content ofa sheet wherein a stock valve is controlled in response to dry fiberweight and the flow rate of fiber through the valve.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of several specific embodiments thereof,especially when taken in conjunction with the accompanying drawings,wherein:

FIG. 1 is a block diagram of a system in accordance with a preferredembodiment of the present invention;

FIG. 2 is a flow diagram indicative of the operations performed by thecomputer of FIG. 1; and

FIG. 3 is a circuit diagram of another embodiment of the apparatus whichcan be included within the computer of FIG. 1.

Reference is now made to FIG. 1 of the drawings wherein there isillustrated a machine for producing a fibrous sheet, such as paper,controlled in accordance with an embodiment of the present invention. Inthe fibrous sheet producing machine, a mixture of water and fiber is fedthrough conduit 11 from a fiber and water source (not shown). Fromconduit 11, the fiber-water mixture is fed through stock valve 12 topump and conduit 13 which is connected to the inlet of headbox 14. Thefiber flow rate through stock valve 12 and conduit 11 is monitored byflow meter 15, the output of which is an electrical signal proportionalin amplitude to the mass flow rate of the mixture flowing from conduit11 through stock valve 12. Because there is a relatively constant andpredetermined percentage of fiber in the mixture fed through conduit 11,the output signal of flow meter 15 at any instant is correlated with thefiber flow rate through stock valve 12.

Downstream of headbox 14 is Fourdrinier wire 16, which receives a jet offiber-water slurry emerging from the headbox slice and removessufiicient water from the slurry in a manner well known to those skilledin the art to form a sheet on the wire. Water removed from the mixtureon wire 16, generally referred to as white water, is fed from catchbasin 17 through conduit 18 into couduit 13, downstream of stock valve12, via pump 10.

The sheet formed on wire 16 is fed to water removing press rollers 19,downstream of which is steam dryer 21. Dryer 21 is heated by steam fromsource 22 fed to the dryer at a controlled pressure through steam valve23. The relatively moisture-free paper sheet emerging from dryer 21 ispolished and smoothed by calender rollers 27. The sheet emerging fromrollers 27 is the finished product that is fed to take-up roll 30.

Between rollers 27 and roll are scanning basis Weight and moisturegauges 2'4 and 25, respectively. Gauges 24 and 25 are respectivelyperiodically scanned by motors 124 and 125 across the entire width ofthe sheet emerging from rollers 27 to derive signals indicative of thetotal weight of the sheet per unit area, a term re ferred to as basisweight, and percentage of moisture in the sheet.

The instantaneous signals derived by gauges 24 and 25 for differentcross-sheet locations of the gauges, as well as the stock flow signalderived by flow meter 15, are fed to computer 26 which derives analogset point signals for stock valve 12 and steam pressure valve 23 onleads 28 and 35, respectively. The stock valve set point signal is fedto difference node 29, where it is compared with a signal on lead 31indicative of the actual position of stock valve 12, as derived fromvalve actuator 32. The resulting difference signal derived by node 29activates automatic controller 33 to drive stock valve 12 to theposition indicated by the set point signal on lead 28. Difference node34 similarly responds to the steam set point signal derived by computer26 on lead 35 and the steam valve 23 position, as indicated by actuator3'7 therefor to activate automatic controller 36 for steam valveactuator 37.

Computer 26 responds to the analog signals derived by gauges 24 and 25,as well as flow meter 15, to actuate stock valve 12 and steam pressurevalve 23 so that a noninteracting control between the sheet fiber orbone dry basis weight and moisture is achieved. To this end, computer 26responds to gauges 24 and 25 to derive signals indicative of the averagebone dry basis weight and moisture content for one scan of gauges 24 and25 across the width of the sheet, i.e., profile average signals for bonedry basis Weight and moisture. The computer responds to these averagevalue signals, as well as the fiber flow rate signal derived from meter15 and preprogrammed signals in a memory thereof to solve Equations 4and 7 for incremental changes in the fiber flow and steam pressurechanges to achieve noninteracting moisture and bone dry basis weightcontrol of the sheet being formed. From the solution of Equations 4 and7, the positions of stock valve 12 and steam pressure valve 23 aredetermined and the set points for these valves are derived on leads 28and 35, respectively.

Computer 26 can take the form of an analog computer, special purposedigital or general purpose digital computer. In a preferred embodimentof the invention actually built and constructed, computer 26 is ageneral purpose digital computer having an analog-todigital anddigital-to-analog input-output elements, as well as the usual memory,arithmetic unit and transfer buses.

The memory of computer 26 is programmed to solve Equations 4 and 7 in astep-by-step manner, described infra in conjunction with FIG. 2. Inaddition, the memory includes prestored values indicative of partialderivative coefiicients for the particular paper making machine for eachgrade and type of paper to be fabricated. The coefiicients aredetermined on an experimental, a priori basis and are commensurate withthe rate of change of change of moisture with respect to bone dry basisweight (BM/BBDBW) and rate of change of mositure with respect to steampressure (EM/8P). For any particular grade or type of paper beingfabricated, the coefficients EM/BP and BM/BBDBW can be considered asconstant and are stored in tabular form in the computer memory andretrieved therefrom in response to signals fed into the computerindicative of the grade and type of paper being formed. The memory ofcomputer 26 includes a sufficient number of bit locations to storeinstantaneous values of total basis weight and moisture as derived fromgauges 24 and 25 as they scan across the sheet. In addition, thecomputer memory includes a listing of initial set point or target valuesfor bone dry basis weight and moisture for each particular grade andtype of paper being formed. Initial set points for stock valve 12 andsteam valve 23 are stored in the memory for each grade and type of paperformed by the machine. Adequate space is provided in the memory forstoring measured values of stock flow through meter 15 to enable theaverage stock flow to be computed with the same periodicity as theaverage moisture and bone dry basis weight quantities. The memory ofcomputer 26 is sufliciently large to store the results of computationsperformed by the arithmetic unit of the computer, and is preloaded withnonlinear table look-up functions relating calculated values of flowthrough stock valve 12 and steam pressure valve 23 to set points for thestock valve and steam pressure valves.

To provide an understanding as to the manner by which computer 26responds to the input signals from gauges 24 and 25 and flow meter 15and stored signals in the memory thereof to solve Equations 4 and *7,attention is now directed to the flow diagram of FIG. 2. The outputsignals of total basis Weight and moisture gauges 24 and 25 areperiodically converted into digital signals as the gauges scan acrossthe sheet and stored in the memory of computer 26. After gauges 24 and25 have been scanned across the width of the sheet, the memory ofcomputer 26 stores a pair of signals for each of a multiplicity oftransverse positions of the sheet, as indicated by blocks 41 and 42,respectively.

The stored values of total basis weight and moisture are combined in thecomputer arithmetic unit to derive a signal indicative of the sheet bonedry basis weight at each transverse sheet position. To this end, eachmoisture signal (M) 42 is subtracted from one in the computer arithmeticunit and the resultant difference is returned to a different memory slotfor each transverse position, an operation indicated by node 43'. Theresultant (l-M) difference signals for each transverse position arecombined with the total basis weight signal (BW) 41 for thecorresponding transverse sheet positions in a multiplicative manner inthe computer arithmetic unit, the output of which is returned to adifferent memory location for each transverse sheet position and isindicative of sheet bone dry basis weight at the different transversepositions, operations indicated in the flow diagram by box 44.

After the bone dry basis weight for each cross sheet position iscalculated and stored in memory, the average bone dry basis weight forthe entire Width of the sheet is calculated, an operation indicated bybox 45. To this end, the different cross sheet position bone dry basisweight signals are read out in seriatim and accumulated in a register inthe computer memory. The accumulated result is divided by the number ofcross sheet positions from which data are taken. A similar averagingoperation is performed on stored moisture signals derived from theseveral cross sheet locations from which data are taken, an operationindicated by box 46. The stored cross sheet or profile averages for bonedry basis weight and moisture, the oper ations of boxes 45 and 46,respectively, are compared with target or set point values for bone drybasis weight and moisture stored in the computer memory and retrieved inresponse to command signals indicative of grade and type of paper beingmanufactured. The comparisons are performed by subtracting thecalculated average values from the stored target values in the computerarithmetic unit, operations indicated by summing nodes 47 and 48,respectively. The difference between the average bone dry basis weightand moisture signals and the set points there for, ABDBW and AM,respectively, are error signals which are returned to appropriate memoryslots.

From the stored, calculated values of AM and ABDBW, as well as thestored coefficients of EM/BBDBW, the change in steam pressure valvesetting, AP, is calculated. To this end, the memory location storingABDBW is multiplied in the computer arithmetic unit with the storedvalue of (AMW ABDBW) 8 signal is returned to the computer memorylocation previously storing AM and is then divided in the arithmeticunit by the coeflicient 'dM/BP stored in memory to derive a AP signal,an operation indicated by box 52. The AP signal remains in a register inthe computer arithmetic unit and is algebraically combined therein witha value for steam dryer pressure previously stored in the computermemory and retrieved therefrom, an operation indicated by box 53. Theresult of operation 53, indicative of desired steam pressure for dryer21 (P), is returned to the same memory location as the one Where theprevious desired steam dryer pressure value was stored. The initialvalue for desired steam pressure is loaded into the selected memorylocation at the beginning of the run for a particular type and grade ofpaper as an initial value for P from a read only portion of memorycontaining a priori determined values thereof. In response to anoperator selecting a particular type and grade of paper to bemanufactured, the a priori detremined value of desired steam pressure isread from the read only section of memory to the memory locationassociated with operation 53. As the machine operations occur, theinitial value of desired steam dryer pressure set into the memorylocation associated with operation 53 is incremented in a positive ornegative direction in response to the AP signal resulting from thedivision operation indicated by box 52.

From the steam pressure indicating signal stored in the memory locationassociated with operation 53, a steam pressure to steam valve positiontable look-up is performed in the memory to derive a digital outputsignal indicative of the set point for steam valve 23, an operationindicated by box 54. The steam valve setting is coupled to adigital-to-analog converter, the output of which is derived on line 35to control the position of valve 23 in a manner described supra.

The set point for stock valve 12 is calculated by computer 26 inaccordance with Equation 4 in response to the stored values in thecomputer memory indicative of calculated average bone dry basis weight,bone dry basis weight set or target point, average stock flow throughstock valve 12 and the previous setting of the stock flow rate. To thisend, the error value of bone dry basis weight, ABDBW, derived duringoperation 47, is retrieved from memory and combined With the value inmemory indicative of the computed average value for fiber flow (6)through stock valve 12 over the same time period as gauges 2.4 and 25scanning across the sheet. This operation indicated by box 55, ismultiplicative and performed by the computer arithmetic unit in timesequence with the operations involved in determining the steam valve setpoint. The product (Q-ABDBW) is fed from the arithmetic unit to a memorylocation of computer 26, subsequently retrieved from memory to thearithmetic unit and divided by the bone dry basis weight previouslycomputed and stored during operation 45. The division operation yieldingthe quotient (ABDBW-Q FFFW is performed by computer 26 in the arithmeticunit thereof and the result is transferred back to the computer memory,operations indicated by box 56. The quotient computed and stored duringoperation 56 is a signal having a magnitude and polarity indicative ofthe change in stock flow (AQ) through valve 12 necessary to providenoninteraction between the moisture and bone dry basis weight of thesheet being formed.

The quotient stored during operation 56 next increments the set point(Q) for the value of fiber flow through stock valve 12 stored by thecomputer memory. The incrementing operation 58 is performed in thecomputer arithmetic unit and the result is returned to the computermemory at the same place as where the previous value for the sheet fibercontent was stored. In the same manner as described supra with regard tosteam pressure, the

initial value for the sheet fiber content for the particular grade andtype of paper being formed is retrieved from a read only section ofmemory and fed into the memory location associated with operation 58.Each new value for flow rate derived during operation 58 is fed to thecomputer memory and utilized therein as an index for a table look-uprelating the stock valve 12 position with the calculated value for fiberflow rate. The resultant of the table look-up operation is transformedto the computer digitalto-analog converter output and is fed via line 28to drive stock valve 12 in the manner indicated supra.

After each scan of gauges 24 and 25 across the width of the sheet,computer 26 responds to a start signal to perform each of the previouslymentioned operations in one second or less. Thereby, the stock and steamvalve signals on leads 28 and 35 are substantially simultaneouslyderived and the stock and steam valves 12 and 23 are substantiallysimultaneously or concomitantly controlled in a coordinated manner sothat changes in the rate at which fiber is fed through stock valve 12 toheadbox 14 do not substantially affect the moisture in the sheet.

Reference is now made to FIG. 3 of the drawings wherein there isillustrated still another embodiment of the system of the presentinvention. In the embodiment of FIG. 3, an analog computer type systemis utilized in place of the digital computer described in conjunctionwith 'FIG. 2. A further distinction between the systems of FIGS. 2 and 3is that the flow rate through stock valve 12 is not monitored andindividual coefficients for the various machine parameters are notprovided. Instead, a number of settings are manually inserted, dependingupon experimental results derived from operating the system.

Referring now more particularly to FIG. 3, the instantaneous signalsderived by basis weight and moisture gauges 24 and 25 are combined toderive a bone dry basis weight signal. To this end, the D.C. analogoutput of moisture gauge 25 (a signal magnitude M) is subtracted from aD.C. voltage indicative of unity in difference node 61, the output ofwhich is fed to analog multiplier 62, also responsive to theinstantaneous output of basis Weight gauge 24 (a signal magnitude BW).The product output of multiplier 62, a D.C. signal proportional to BDBW:BW (1M), is fed to profile averaging computer 63.

Profile averaging computer 63 responds to the instantaneous bone drybasis weight input signal thereof for the duration of a scan of gauges24 and 25 across the Width of the sheet and upon completion of the gaugescan, derives a constant output signal, in the form of a shaft position.The rotational position of the shaft is indicative of the average valueof bone dry basis weight for the scan. Similarly, the output of moisturegauge 25 is averaged over a scan of the gauges across the Width of thesheet being manufactured by profile averaging computer 68. Upon thecompletion of each scan of gauges 24 and 25 across the sheet, profileaveraging computer 68 produces a shaft rotation having a positioncommensurate with the average value of moisture gauge 25 while the gaugeis scanning across the sheet.

The shaft rotation outputs of profile averaging computers 63 and 68respectively drive sliders 65 and 69 of potentiometers 66 and 71,included in bridges 67 and 72. Bridges 67 and 72 are driven by floatingD.C. power supplies 73 and 74, respectively, and include target or setpoint potentiometers 75 and 76. Sliders 77 and 78 of potentiometers 75and 76 are both grounded and manually set to positions correspondingwith set point, i.e., target values for bone dry basis weight andmoisture, respectively.

To adjust the range of voltages applied to bridges 67 and 72 by powersupplies 73 and 74, each bridge includes potentiometer 79 and 80 havingmanually adjusted sliders. The slider settings of potentiometers 80 areadjusted in accordance with the low range of bone dry basis weight andmoisture for a particular grade of paper being fabricated, while thesettings of potentiometers 79 in the respectively indicative of bone drybasis weight error and moisture error are combined linearly in summingnode 84. To this end, the D.C. voltages at sliders 65 and 69 are fedthrough variable gain D.C. amplifiers 85 and 86, set in accordance withthe desired degree of compensation to provide noninteracting bone drybasis weight and moisture control. The gain of amplifier 85 correspondsdirectly with the amount of compensation required to attainnoninteracting bone dry basis weight and moisture control of the sheetbeing manufactured. The gain setting of amplifier 85 is dependent uponthe relationship between changes in moisture and fiber content for thetype and grade of paper made by each particular paper machine, as wellas the transport lags from stock valve 12 and dryer 21 to the locationof gauges 24 and 25. These parameters are determined on an empiricalbasis to control the gain of the amplifier in such a manner as toprovide the desired results. While the gain of amplifier 85 is adjustedto enable an output signal of sufiicient magnitude to be derived toachieve a one-to-one relationship between bone dry basis weight andmoisture compensation. Without amplifier 85, the range of values betweenthe bone dry basis weight and moisture compensation is limited toapproximately 0.2 to 1.

With the system in normal operating condition, the outputs of amplifiers85 and 86 are added together in summing node 84, the output of whichdirectly drives automatic controller 36 for steam valve 23. In normaloperation, contacts 87 of relay 88 are closed in response toenergization of relay coil 89 by the closure of manual switch 90 whichconnects A.C. supply 91 to coil 89.

Switch 90 is activated to the open circuit condition to openv contacts87 only when the paper machine is in a start-up condition, while a gradechange is being performed or if something appears to be malfunctioningin the interaction control system.

The output voltage of summing node 84 is applied directly to theautomatic controller 36 for valve 23 because it is a measure of theactual deviation of the pressure change of the steam valve, therebyeliminating set point comparator 34 of FIG. 1. In a simliar manner,stock valve- 12 is controlled in response to the deviation of measuredbone dry basis weight relative to the target value thereof, as monitoredby the voltage between potentiometer slider 65 and grounded slider 77 ofbridge 67. To

this end, the voltage at slider 76 is fed directly to automaticcontroller 33 and set point comparator 29 is not employed.

While there have been described and illustrated several 'specificembodiments of the invention, it will be clear that variations in thedetails of the embodiments specifically illustrated and described may bemade without departing from the true spirit and scope of the invention.For example, flow meter 15 can be eliminated in systems wherein theposition of valve 12 can be accurately correlated with the actual fiberflow rate through conduit 11. In addition, the position controllers forvalves 12 and 23 can be replaced by feedback controllers responsive tomeasurements of stock flow through valve 12 and the pressure of steamfed by source 22 to the dryer. In such instances, flow meter 15 and apressure transducer in the line between source 22 and dryer 21 areprovided and the outputs thereof are combined with target values forstock flow and steam pressure calculated by computer 26 to derive errorsignals that control stock valve 12 and steam valve 23.

A further possible change, having particular application with regard tofeedback control of valve 12 in response to an error signal derived inresponse to the output of flow meter 15 and a calculated value for fiberfiow rate,

involves determining the value of AQ in response to the set points forBDBW and Q, rather than the calculated values of BDBW and Q Since theset points for BDBW and Q do not generally deviate by a great amountfrom the calculated average values thereof, the value of AQ determinedin this manner is frequently sufficiently accurate for control with thepresent system. Using the set points for BDBW and Q in determining thevalue of AQ has the advantage of faster settling time when the system isstarting or while a grade change is taking place. In systems whereinstock flow error is calculated, the steam pressure target value can becomputed in accord ance with rather than Equation 7 supra. Theequivalency between Equations 7 and 8 results from the equality of OM aB D BW AB DB W Still a further possible change to the system is that thevalues of bone dry basis weight moisture and total basis weight measuredor calculated at the different cross-sheet zones need not be stored forthe entire period while the gauge is scanning across the sheet. Inparticular, the value of total basis weight for each zone can bediscarded after bone dry basis weight for the zone has been calculated.The values of bone dry basis weight and moisture for the several zonescan be accumulated as the gauges scan across the sheet and the totalvalues thereof divided by the number of zones after the scan has beencompleted. If it is desired to average out certain property changes inthe sheet along its length, the average property values from severalscans can be averaged together to derive the moisture and bone dry basisweight error signals, rather than averages derived from a single scan ofthe gauges.

I claim:

1. A system for controlling the moisture and fiber content of a fibroussheet formed by feeding a mixture of Water and fiber to a fibrous sheetforming machine including a dryer comprising measuring means forderiving a first signal indicative of the dry fiber weight of a formedsheet, measuring means for deriving a second signal indicati-ve of themoisture content of the sheet downstream of the dryer, means combiningsaid first and second signals for deriving a dryer control signal, meansresponsive to the first signal for deriving a control signal for therate of fiber flow to the machine concomitantly with the derivation ofthe dryer control signal, means for controlling the drying rate of thedryer in response to said dryer control signal, and means forcontrolling the rate of fiber flow to the machine responsive to saidfiber flow rate control signal.

2. The system of claim 1 further including measuring means for derivinga third signal indicative of the rate of fiber flow to the machine, andmeans responsive to said first and third signals for deriving saidcontrol signal for the rate of fiber flow to the machine.

3. The system of claim 2 wherein said dryer control signal derivingmeans includes means responsive to errors between the measured fiberweight and moisture content of the sheet and target values therefor.

4. The system of claim 3 wherein said fiber flow rate control signalderiving means includes means responsive to errors only between themeasured fiber weight of the sheet and a target value therefor.

5. The system of claim 1 wherein said dryer control signal derivingmeans includes means responsive to errors between the measured fiberweight and moisture content of the sheet and target values therefor.

6. The system of claim 5 wherein said fiber flow rate control signalderiving means includes means responsive to errors only between themeasured fiber weight of the sheet and a target value therefor.

7. The system of claim 1 wherein said fiber flow rate control signalderiving means includes means responsive to errors only between themeasured fiber weight of the sheet and a target value therefor.

8. The system of claim 1 wherein said fiber content measuring meansincludes a moisture gauge and a total basis weight gauge scanning acrossat least a portion of the sheet width together, means responsive to thescanning gauges at corresponding cross sheet portions to derive ameasure of dry fiber Weight for individual cross sheet portions, meansresponsive to the scanning gauge responsive means and the scannedmoisture gauge for separately averaging the fiber weight and moisturecontent of the sheet for a predetermined length across the sheet width.

9. A method of controlling the fiber and moisture content of a fibroussheet formed by feeding a mixture of water and fiber to a fibrous sheetforming machine including a dryer comprising providing a measurement ofthe moisture content of the sheet downstream of the dryer, providing ameasurement of the dry fiber weight of the formed sheet, providing adryer control signal which is a function of both said moisture and dryfiber weight measurements, and controlling the dryer in response to saiddryer control signal concomitantly with controlling the rate at whichfiber is fed to the machine in a coordinated manner so that changes inthe rate at which fiber is fed to the machine do not substantiallyaifect the moisture content of the sheet.

References Cited UNITED STATES PATENTS 3,073,153 1/1963 Petitjean 73-73FOREIGN PATENTS 911,975 12/ 1962 Great Britain 73-73 OTHER REFERENCESOtt, R. N.: The A-B-C System for Moisture and Basis Weight Contro, PaperTrade Journal, Mar. 24, 1958, pp. 3033.'

Dahlin, E. B. et al.: Designing and Tuning Digital Controllers,Instruments and Control Systems, July, 1968, pp. 87-91.

Roberts: Some Plain Talk on Digital Computers, Pulp & Paper (Aug. 12,1968), pp. 32-7.

Thompson The Paper Machine Under Digital Com- 1;121te9r Control, ThePaper Industry (May 1962), pp.

S. LEON BASHORE, Primary Examiner A. DANDREA, JR., Assistant ExaminerUS. Cl. X.R.

